The invention relates to receiving spread spectrum radio signals, such as digitally modulated signals in a Code Division Multiple Access (CDMA) mobile radio telephone system, and more particularly, to configuring a RAKE receiver.
FIG. 1 illustrates the use of base stations to transmit radio waves to mobile users (mobile stations) in a cellular system 10. Base station 30 transmits a signal 40 that has a maximum signal strength that is limited so as to reduce interference with other base stations. The maximum signal strength of the base station""s transmission creates a foot print or a region within which mobile stations 50 and 60 can communicate with base station 30. If base station 30 uses a single omni-directional antenna, the foot print extends in an unlimited direction (360 degrees). While each footprint is an irregular shape that overlaps with adjacent foot prints, a foot print is often depicted as a hexagon 20 and is usually referred to as a cell.
In most systems, the base station 30 transmits a broadcast signal that is transmitted to all the mobile stations in cell 20. The mobile stations use different traffic signals, but the same broadcast channel. The broadcast signal contains, for example, paging messages that are needed by all the mobile stations in the cell. The base station can control the power of each traffic signal, but the broadcast signal has to be able to reach as far as the cell""s border. Therefore, the broadcast channel usually contains more signal power than the individual traffic channels.
FIG. 2 is a schematic diagram of an example of a CDMA system. A transmitter 30 can transmit input user data to multiple users. In a traditional CDMA system, each symbol of input user data 31 is multiplied by a short code or chip sequence 33. There is a unique short code for each input user. Input user data is then spread by a long code or chip sequence 35. While the short codes eliminate multiple access interference among users in the same cell, the long code is used to eliminate multiple access interference among the transmitters. An accumulator 36 adds the spread signals to form a composite signal 37. Composite signal 37 is used to modulate a radio frequency carrier 38 which is transmitted by a transmitting antenna 39.
A receiver 50 has a receiving antenna 59 for receiving signal 40. Receiver 50 uses a carrier signal 58 to demodulate signal 40 and to obtain composite signal 58. Composite signal 57 is multiplied by a synchronized long code or chip sequence 55. Long code 55 is a locally generated complex conjugated replica of long code 35.
The despread signal 54 is then multiplied by a synchronized short code or chip sequence. Short code 53 is a locally generated complex conjugated replica of short code 33 (or one of the other N short codes used by transmitter 30). The multiplication by short code 53 suppresses the interference due to transmission to the other users. A digital logic circuit 52 (e.g., a summation and dump unit) can be used to provide an estimate of input user data 31.
It will be evident to those skilled in the art that receiver 50 can not reconstruct input user data 31 unless it can (1) determine long code 35 and synchronize a locally generated complex conjugated replica of long code 35 with the received signal 57, and (2) determine short code 33 and synchronize a locally generated complex conjugated replica of short code 33 with the despread signal 54. It is for this reason that many CDMA signals contain a pilot signal or a periodic code (synchronization code). The synchronization codes can be found by using a matched filter or a correlation scheme and by identifying the correlation peaks.
FIG. 3 is a schematic diagram of an exemplary frame structure. Channel 40 has multiple frames 42. Each frame 42 has a constant number of slots 44. Each slot 44 contains one or more pilot symbol(s) 46. The long code 35 is repeated each frame so that, for example, the first pilot symbol in each frame is multiplied by the same portion of long code 35, and successive pilot symbols are multiplied by the same successive portions of long code 35. While the receiver can use the pilot signal to synchronize the received signal and search for multipath delays, in some systems, the pilot signal is a relatively small portion of each frame and does not contain much energy. A broadcast channel may use the same, or a different, frame structure. The broadcast channel may contain a pilot signal that is considerably longer. In either case, the broadcast channel usually contains more energy than a traffic channel.
FIG. 4a illustrates the use of three directional antennas to divide a cell into three 120xc2x0 sectors. Cell 20 has three sectors 21, 22, and 23. FIG. 4b illustrates the use of six directional antennas to divide a cell into six 60xc2x0 sectors. Cell 20 has six sectors 21, 22, . . . , and 26. As discussed above, the long code 55 suppresses the interference due to other transmitters, and the short code 53 suppresses the interference due to other users. However, as the number of users increases so does the interference. In some systems, it is necessary to use directional antennas to subdivide each cell.
If base station 10 uses directional antennas, base station 10 can transmit multiple signals to smaller groups. When a base station uses directional antennas, each directional antenna transmits to a smaller number of mobile stations than a single antenna would. As a result, the amount of interference decreases and the base station can support a larger number of mobile stations without exceeding an acceptable level of interference noise. If each of the mobile stations uses the same broadcast channel, the base station can use an omnidirectional antenna to transmit the broadcast signal, and directional antennas to transmit the traffic signals.
In mobile communication systems, signals transmitted between base and mobile stations typically suffer from echo distortion or time dispersion (multipath delay). Multipath delay is caused by, for example, signal reflections from large buildings or nearby mountain ranges. The obstructions cause the signal to proceed to the receiver along not one, but many paths. The receiver receives a composite signal of multiple versions of the transmitted signal that have propagated along different paths (referred to as xe2x80x9craysxe2x80x9d). The rays have different and randomly varying delays and amplitudes.
Each distinguishable xe2x80x9crayxe2x80x9d has a certain relative time of arrival, Tn seconds. A receiver can determine the relative time of arrival of each ray by using a matched filter, a search finger that is shifted, or any other correlation scheme. The output of the matched filter or the correlation scheme is usually referred to as the multipath profile (or the delay profile). Because the received signal contains multiple versions of the same signal, the delay profile contains more than one spike.
FIG. 5 is an example of a multipath profile. The ray that propagates along the shortest path arrives at time To with amplitude A0, and rays propagating along longer paths arrive at times T1, T2, . . . , TN with amplitudes A1, A2, . . . , AN, respectively. In order to optimally detect the transmitted signal, the spikes must be combined in an appropriate way. This is usually done by a RAKE receiver, which is so named because it xe2x80x9crakesxe2x80x9d different paths together. A RAKE receiver uses a form of diversity combining to collect the signal energy from the various received signal paths (or rays). The term xe2x80x9cdiversityxe2x80x9d refers to the fact that a RAKE receiver uses redundant communication channels so that when some channels fade, communication is still possible over non-fading channels. A CDMA RAKE receiver combats fading by detecting the echo signals individually, and then adding them together coherently.
FIG. 6 is a schematic diagram of a RAKE receiver with four fingers. A radio frequency (RF) receiver 110 demodulates a received signal and quantizes the demodulated signal to provide input signal 112. Each finger uses input signal 112 to recover signal power from a different path. The receiver can use a searcher to find a set of signal paths.
Using the example in FIG. 5, the searcher determines that the peak at T900, has the greatest amplitude. Because this path is the strongest path, one of the fingers, for example, finger 320 is configured to receive a path having a delay of T900. The receiver can be configured by, for example, delaying digital samples 112 by T900, or by shifting chip sequence(s) 321 by an equivalent amount.
Similarly, input signal 112 can be correlated in finger 322 with a chip sequence 323 that has a phase corresponding to T800; in finger 330 with a chip sequence 331 that has a phase corresponding to T750; and in finger 322 with chip sequence(s) having a phase corresponding to T850. The finger outputs are multiplied by individual weights 340, 342, 350, and 352 to maximize the received signal-to-noise-and-interference ratio. The weighted outputs are then added by an accumulator 362. The output of the accumulator 362 is fed to a threshold device 364, or to a quantizer that outputs soft information.
It is important that the RAKE receiver use the strongest taps (paths) for each finger. If the receiver does not use the strongest taps, the receiver will ask for more power and thereby increase the interference experienced by the other receivers. The overall interference is minimized when each of the receivers uses the least amount of power possible.
Using a searcher is costly and computationally complex. It is not only time-consuming, it also decreases the battery life of hand-held units. However, if the receiver does not find the strongest set of taps, the overall performance of the system will decline. Because the strength of the taps is important to the performance of the system, and the amount of search time is important to the performance of the receiver, there is a need for a RAKE receiver that can generate a strong set of taps in a shorter amount of time.
These and other drawbacks, problems, and limitations of conventional RAKE receivers are overcome by obtaining information from a searcher that has searched a first channel, and using the information to search a second channel. In a preferred embodiment, a first searcher searches a broadcast channel, a second searcher searches a traffic channel, and the second searcher uses information from the first searcher to search the traffic channel. As a result, the second searcher can generate a strong set of taps for the traffic channel in a shorter amount of time. The searchers can use a matched filter, a search finger that is shifted, or any other correlation scheme.
According to one aspect of the invention, a first searcher generates a delay profile for a first channel (e.g., a broadcast channel), and a second searcher uses the. delay profile to generate a set of taps for a second channel (e.g., a traffic channel).
According to another aspect of the invention, a first searcher is configured to find a maximum correlation value (or a set of maximum correlation values), and a second searcher shifts a search finger by an amount equal to the delay corresponding to the maximum correlation value(s). The second searcher can also shift a search finger by an amount equal to the sum of the delay corresponding to the maximum correlation value(s) and a predetermined value.
According to another aspect of the invention, a first searcher is configured to find a maximum correlation value (or a set of maximum correlation values). A second searcher uses the maximum correlation value or values to generate a search window for a search finger or a matched filter.
According to another aspect of the invention, a first searcher is configured to generate a delay profile for a first signal and the second searcher uses a minimum threshold value to generate a search window for a search finger or a matched filter. The second searcher selects a start delay value and a finish delay value that correspond to correlation values exceeding the minimum threshold value. The searcher can search phases between the start delay value and the finish delay value.
According to another aspect of the invention, a second searcher is designed to determine whether to use information from a first searcher. The second searcher can make this decision by observing a delay profile found by the first searcher for a first channel and the delay profile found by the second searcher for a second channel. Or alternatively, the second searcher can make this decision by processing information from the source of the first channel.
An advantage of the invention is that the receiver can generate a strong set of taps in a shorter amount of time. Another advantage is that the receiver can receive signals sooner, and with fewer computations. Another advantage is that the battery life of hand-held units is increased.
The invention is particularly advantageous when the first signal is a broadcast channel, and the second signal is a traffic channel. Generally speaking, the broadcast channel contains more energy than the traffic channel, and the mobile station can find a better set of taps for the traffic channel in a shorter amount of time. In some cases, the broadcast channel is transmitted by a different antenna than the traffic channel. The mobile station can be configured to make a decision whether to use information from the broadcast channel to search the traffic channel.